U.S. patent application number 12/288861 was filed with the patent office on 2009-07-02 for method for making thermionic electron source.
This patent application is currently assigned to Tsinghua University. Invention is credited to Shou-Shan Fan, Kai-Li Jiang, Liang Liu, Peng Liu.
Application Number | 20090170394 12/288861 |
Document ID | / |
Family ID | 40799052 |
Filed Date | 2009-07-02 |
United States Patent
Application |
20090170394 |
Kind Code |
A1 |
Liu; Peng ; et al. |
July 2, 2009 |
Method for making thermionic electron source
Abstract
A method for making a thermionic electron source includes the
following steps: (a) supplying a substrate; (b) forming a first
electrode and a second electrode thereon; and (c) spanning a carbon
nanotube film structure on a surface of the first electrode and the
second electrode with a space defined between the thermionic
emitter and the substrate.
Inventors: |
Liu; Peng; (Beijing, CN)
; Liu; Liang; (Beijing, CN) ; Jiang; Kai-Li;
(Beijing, CN) ; Fan; Shou-Shan; (Beijing,
CN) |
Correspondence
Address: |
PCE INDUSTRY, INC.;ATT. Steven Reiss
458 E. LAMBERT ROAD
FULLERTON
CA
92835
US
|
Assignee: |
Tsinghua University
Beijing City
CN
HON HAI Precision Industry CO., LTD.
Tu-Cheng City
TW
|
Family ID: |
40799052 |
Appl. No.: |
12/288861 |
Filed: |
October 23, 2008 |
Current U.S.
Class: |
445/49 |
Current CPC
Class: |
H01J 31/127 20130101;
H01J 9/04 20130101; H01J 1/14 20130101 |
Class at
Publication: |
445/49 |
International
Class: |
H01J 9/08 20060101
H01J009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 29, 2007 |
CN |
200710125673.X |
Claims
1. A method for making a thermionic electron source, the method
comprising the following steps: (a) supplying a substrate; (b)
forming a first electrode and a second electrode on the substrate;
and (c) spanning a carbon nanotube film structure on a surface of
the first electrode and the second electrode with a space defined
between the thermionic emitter and the substrate.
2. The method as claimed in claim 1, wherein step (b) is executed
by a method selected from a group consisting of a screen-printing
method, an offset printing method, an electrostatic spraying
method, an electrophoresis method, a lithography coating method,
and a UV-curing method.
3. The method as claimed in claim 1, wherein step (b) comprises the
following steps: (b1) supplying a conductive slurry; (b2) coating
the conductive slurry on a surface of the substrate according to a
predetermined pattern; and (b3) heat treating the substrate with
the conductive slurry thereon, thereby acquiring the first
electrode and the second electrode.
4. The method as claimed in claim 3, wherein the conductive slurry
comprises conductive materials, adhesives, organic solvent and
organic additive.
5. The method as claimed in claim 4, wherein the conductive
material is selected from a group consisting of silver, gold, and
copper; the adhesive can be selected from a group consisting of
inorganic adhesive, organic adhesive, and low melting point metals;
and a weight ratio of the conductive slurry and the adhesive
approximately ranges from 0.1:10 to 10:1.
6. The method as claimed in claim 4, wherein the organic solvent is
selected from a group consisting of ethanol, glycol, hydrocarbons,
water, and the mixture thereof.
7. The method as claimed in claim 4, wherein the organic additive
is selected from a group consisting of tackifying agent,
dispersants, plasticizers, and surface-active agent.
8. The method as claimed in claim 4, wherein step (b3) is executed
by heat treating the substrate with the conductive slurry thereon
to remove organic ingredients therein, and cooling the conductive
slurry, thereby forming the first electrode and the second
electrode on the substrate.
9. The method as claimed in claim 8, wherein a temperature of the
heat treatment is lower than or equal to 600.degree. C.
10. The method as claimed in claim 1, wherein step (c) comprises
the following steps: (c1) forming at least one carbon nanotube
film; and (c2) placing the at least one carbon nanotube film on the
first electrode and the second electrode.
11. The method as claimed in claim 10, wherein step (c1) comprises
the following steps: (c11) providing an array of carbon nanotubes;
and (c12) pulling out a carbon nanotube film from the array of
carbon nanotubes with a tool.
12. The method as claimed in claim 10, wherein step (c2) is
executed by covering the carbon nanotube film on the surface of the
first electrode and the second electrode along a direction
extending from the first electrode to the second electrode.
13. The method as claimed in claim 10, wherein step (c2) is
executed by covering at least two carbon nanotube films stacked
with each other and situated such that a preferred orientation of
the carbon nanotubes is set at an angle with respect to each other,
the angle approximately ranging from 0.degree. to 90.degree..
14. The method as claimed in claim 10, wherein step (c2) is
executed by the following steps: (c21) supplying a supporting
element; (c22) stacking at least two carbon nanotube films and
being situated such that a preferred orientation of the carbon
nanotubes of one of the films is set at an angle with respect to
each other to form a carbon nanotube film structure, the angle
approximately ranging from 0.degree. to 90.degree.; (c23) cutting
away excess portion of the carbon nanotube film structure; (c24)
treating the carbon nanotube film structure via an organic solvent;
(c25) removing the carbon nanotube film structure from the
supporting element to form a free-standing carbon nanotube film
structure; and (c26) using the free-standing carbon nanotube film
structure as the carbon nanotube film structure.
15. The method as claimed in claim 10, further comprising treating
the carbon nanotube film structure with an organic solvent.
16. The method as claimed in claim 15, wherein the carbon nanotube
film structure is treated by either applying the organic solvent
from soak the entire surface of the carbon nanotube film structure
or immerging the carbon nanotube film structure in a container
filled with the organic solvent.
17. The method as claimed in claim 16, wherein the organic solvent
is volatilizable and can be selected from the group consisting of
ethanol, methanol, acetone, dichloroethane, chloroform, and
combinations thereof.
18. The method as claimed in claim 1, further comprising a step of
coating a conductive glue on the surface of the first electrode and
the second electrode.
19. The method as claimed in claim 1, further comprising a step of
forming at least one fixing element on the surface of the first
electrode and the second electrode by a screen printing method,
offset printing method, electrostatic spraying method,
electrophoresis method, lithography coating method or a UV-curing
method to secure the carbon nanotube film structure on the surface
of the first electrode and the second electrode.
Description
RELATED APPLICATIONS
[0001] This application is related to commonly-assigned
applications entitled, "THERMIONIC ELECTRON SOURCE", filed ______
(Atty. Docket No. US18568); "THERMIONIC EMISSION DEVICE", filed
______ (Atty. Docket No. US18570); "THERMIONIC EMISSION DEVICE",
filed ______ (Atty. Docket No. US18571); "THERMIONIC ELECTRON
EMISSION DEVICE AND METHOD FOR MAKING THE SAME", filed ______
(Atty. Docket No. US18569); and "THERMIONIC ELECTRON SOURCE", filed
______ (Atty. Docket No. US17306).
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to a method for making a
thermionic electron source adopting carbon nanotubes.
[0004] 2. Discussion of Related Art
[0005] Carbon nanotubes (CNT) are a carbonaceous material and have
received much interest since the early 1990s. Carbon nanotubes have
interesting and potentially useful electrical and mechanical
properties. Due to these and other properties, CNTs have become a
significant contributor to the research and development of electron
emitting devices, sensors, and transistors, among other
devices.
[0006] Generally, an electron-emitting device has an electron
source using a thermal or cold electron source. The thermal
electron source is used by heating an emitter to increase the
kinetic energy of the electrons in the emitter. When the kinetic
energy of the electrons therein is large enough, the electrons will
emit or escape from the emitters. These electrons emitted from the
emitters are thermions. The emitters emitting the thermions are
named thermionic emitters.
[0007] Conventionally, the thermionic electron source includes a
thermionic emitter and two electrodes. The two electrodes are
located on a substrate. The thermionic emitter is located between
two electrodes and electrically connected thereto. The thermionic
emitter is generally made of a metal, a boride or an alkaline earth
metal carbonate. The thermionic emitter can be divided into two
types, a direct-heating type and an indirect-heating type. The
thermionic emitter of the direct-heating type uses a metal ribbon
or a metal thread as the thermionic emitter. The metal ribbon or
metal thread is fixed between the two electrodes by welding. During
use, a voltage is applied between the two electrodes to heat the
metal ribbon or metal thread. Kinetic energy of the electrons in
the metal ribbon or metal thread is increased. When the kinetic
energy of the electrons therein is large enough, thermions will
emit or escape from the emitters. The thermionic emitter of the
indirect-heating type uses a boride or an alkaline earth metal
carbonate as a material of the thermionic emitter. The boride or
alkaline earth metal carbonate is dispersed in a conductive slurry,
and the conductive slurry is directly coated or sprayed on a
heater. The heater can be secured between the two electrodes as a
thermionic emitter. During use, a voltage is applied between the
two electrodes to heat the thermionic emitter. Kinetic energy of
the electrons in the thermionic emitter is increased. When the
kinetic energy of the electrons therein is large enough, thermions
will emit or escape from the emitters. However, the size of the
thermionic emitter using the metal, boride or alkaline earth metal
carbonate is large, thereby limiting its application in
micro-devices. Furthermore, the coating formed by direct coating or
spraying the metal, boride or alkaline earth metal carbonate has a
high resistivity, and thus, the thermionic electron source using
the same has a greater power consumption and is therefore not
suitable for applications involving high current density and
brightness.
[0008] What is needed, therefore, is a method for making a
thermionic electron source having excellent thermal electron
emitting properties and wearability, and can be used in flat panel
displays with high current density and brightness, logic circuits,
and other fields of thermal electron source.
SUMMARY
[0009] In one embodiment, a method for making a thermionic electron
source includes the following steps: (a) supplying a substrate; (b)
forming a first electrode and a second electrode thereon; and (c)
spanning a carbon nanotube film structure on a surface of the first
electrode and the second electrode with a space defined between the
thermionic emitter and the substrate.
[0010] Other novel features and advantages of the present method
for making a thermionic electron source will become more apparent
from the following detailed description of exemplary embodiments
when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Many aspects of the present method for making a thermionic
electron source can be better understood with references to the
following drawings. The components in the drawings are not
necessarily drawn to scale, the emphasis instead being placed upon
clearly illustrating the principles of the present method for
making a thermionic electron source.
[0012] FIG. 1 is a flow chart of a method for making a thermionic
electron source, in accordance with the present embodiment
[0013] FIG. 2 is an exploded, isometric view of a thermionic
electron source in accordance with the present embodiment.
[0014] Corresponding reference characters indicate corresponding
parts throughout the views. The exemplifications set out herein
illustrate at least one embodiment of the present method for making
a thermionic electron source, in at least one form, and such
exemplifications are not to be construed as limiting the scope of
the invention in any manner.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0015] References will now be made to the drawings to describe, in
detail, embodiments of the present method for making a thermionic
electron source.
[0016] Referring to FIG. 1, a method for making a thermionic
electron source includes the following steps: (a) supplying a
substrate; (b) forming a first electrode and a second electrode on
the substrate; and (c) spanning a carbon nanotube film structure on
a surface of the first electrode and the second electrode with a
space defined between the thermionic emitter and the substrate.
[0017] In step (a), the substrate can be made of ceramics, glass,
resins, or quartz, among other insulative materials. A size and
shape of the substrate can be set as desired. In the present
embodiment, the substrate is a glass substrate.
[0018] In step (b), a thickness of the first electrode and the
second electrode approximately ranges from 1 micrometer to 2
millimeters. A distance between the first electrode and the second
electrode approximately ranges from 50 micrometers to 1 millimeter.
The first electrode and the second electrode can be formed on the
substrate by a method selected from a group consisting of a
screen-printing method, an offset printing method, an electrostatic
spraying method, an electrophoresis method, a lithography coating
method, and a UV-curing method. The first electrode and the second
electrode can be formed by coating conductive glue on a surface of
the substrate to secure the first electrode and the second
electrode thereon. In the present embodiment, the first electrode
and the second electrode are formed by a screen-printing
method.
[0019] Step (b), executed by the screen-printing method, further
includes the following substeps: (b1) supplying a conductive
slurry; (b2) coating the conductive slurry on the surface of the
substrate according to a predetermined pattern; and (b3) heat
treating the substrate with the conductive slurry thereon, thereby
forming the first electrode and the second electrode.
[0020] In step (b1), the conductive slurry includes conductive
materials, adhesives, organic solvent and organic additive. The
conductive material can be selected from a group consisting of
gold, silver, copper and other conductive metal. The adhesive can
be selected from a group consisting of inorganic binder, organic
binder, and low melting point metals. The inorganic binder includes
glass powder, silane and water glass. The organic binder includes
fiber resins, acrylic resins, and ethylene resin. The adhesive can
adhere conductive particles together, and adhere conductive slurry
on the surface of the substrate. A weight ratio of the conductive
slurry and the adhesive approximately ranges from 0.1:10 to
10:1.
[0021] The organic additive includes a tackifying agent,
dispersants, plasticizers, or a surface-active agent. The
plasticizers can be selected from a group consisting of grass
diethyl, and butyl ether. The organic solvent can be selected from
a group consisting of ethanol, glycol, hydrocarbons, water, any
other traditional solvents and the mixture thereof. Additionally,
the organic solvent and organic additive can adjust the properties
of the conductive slurry, such as viscosity, fluidity, dry speed,
and other physical properties, and thus, it is conducive to coat
the conductive slurry on the substrate. An amount of the organic
solvents and additives can be adjusted according to the printing
process. The conductive slurry can be placed into a stirring device
to uniformly mix the ingredients thereof.
[0022] In the present embodiment, the conductive slurry includes a
weight ratio of 75% silver, 21% adhesives, 3% low melting glass
powder, and 2% ethanol. The adhesives are a solution of ethyl
cellulose dispersed in terpineol. The conductive glurry is placed
in a three-roll roller mill to uniformly distribute the ingredients
thereof.
[0023] Step (b3) can be executed in an atmosphere or an environment
with oxidized gas therein. A temperature of the heat treatment can
be set according to organic ingredients of the conductive slurry.
Generally, the temperature of the heat treatment is lower than
600.degree. C. The heat treatment is used to form a good mechanical
and electrical contact between the first electrode, the second
electrode, and the substrate.
[0024] In the present embodiment, step (b3) can be executed by the
following steps. Firstly, the conductive slurry is heated from
20.degree. C. for 10 minutes, the temperature of the conductive
slurry reaching up to 120.degree. C. Then the temperature is held
for 10 minutes to remove the terpineol and ethanol in the
conductive slurry. Secondly, the conductive slurry is heated for 30
minutes, the temperature of the conductive slurry reaching up to
350.degree. C. Then the temperature is held for 30 minutes to
remove the ethyl cellulose. Thirdly, the conductive slurry is
heated for 30 minutes, the temperature of the conductive slurry
reaching up to 460-580.degree. C. Then the temperature is held for
30 minutes to closely combine the conductive slurry and the
substrate. Finally, the conductive slurry is naturally cooled,
thereby a first electrode and a second electrode are separately
formed on the substrate.
[0025] Step (c) can be executed by the following steps: (c1)
forming at least one carbon nanotube film; and (c2) placing the at
least one carbon nanotube film on the first electrode and the
second electrode to form a carbon nanotube film structure.
[0026] In step (c1), the method for making the carbon nanotube film
includes the following steps: (c11) providing an array of carbon
nanotubes, specifically, providing a super-aligned array of carbon
nanotubes; (c12) pulling out a carbon nanotube film from the array
of carbon nanotubes, by using a tool (e.g., adhesive tape, pliers,
tweezers, or another tool allowing multiple carbon nanotubes to be
gripped and pulled simultaneously).
[0027] In step (c11), a given super-aligned array of carbon
nanotubes can be formed by the following substeps. Firstly, a
substantially flat and smooth substrate is provided. Secondly, a
catalyst layer is formed on the substrate. Thirdly, the substrate
with the catalyst layer thereon is annealed in air at a temperature
approximately ranging from 700.degree. C. to 900.degree. C. for
about 30 to 90 minutes. Fourthly, the substrate with the catalyst
layer thereon is heated to a temperature approximately ranging from
500.degree. C. to 740.degree. C. in a furnace with a protective gas
therein. Fifthly, a carbon source gas is supplied to the furnace
for about 5 to 30 minutes, and the super-aligned array of carbon
nanotubes is grown on the substrate.
[0028] The substrate can be a P-type silicon wafer, an N-type
silicon wafer, or a silicon wafer with a film of silicon dioxide
thereon. Preferably, a 4-inch P-type silicon wafer is used as the
substrate. The catalyst can be made of iron (Fe), cobalt (Co),
nickel (Ni), or any alloy thereof. The protective gas can be made
up of at least one of nitrogen (N.sub.2), ammonia (NH.sub.3), and a
noble gas. The carbon source gas can be a hydrocarbon gas, such as
ethylene (C.sub.2H.sub.4), methane (CH.sub.4), acetylene
(C.sub.2H.sub.2), ethane (C.sub.2H.sub.6), or any combination
thereof.
[0029] The super-aligned array of carbon nanotubes can be
approximately 200 to 400 microns in height and include a plurality
of carbon nanotubes parallel to each other and substantially
perpendicular to the substrate. The carbon nanotubes in the array
can be selected from a group consisting of single-walled carbon
nanotubes, double-walled carbon nanotubes, or multi-walled carbon
nanotubes. A diameter of the single-walled carbon nanotubes
approximately ranges from 0.5 to 50 nanometers. A diameter of the
double-walled carbon nanotubes approximately ranges from 1 to 10
nanometers. A diameter of the multi-walled carbon nanotubes
approximately ranges from 1.5 to 10 nanometers.
[0030] The super-aligned array of carbon nanotubes formed under the
above conditions is essentially free of impurities, such as
carbonaceous or residual catalyst particles. The carbon nanotubes
in the super-aligned array are closely packed together by the van
der Waals attractive force.
[0031] Step (c12) can be executed by selecting a plurality of
carbon nanotube segments having a predetermined width from the
array of carbon nanotubes, and pulling the carbon nanotube segments
at an even/uniform speed to achieve a uniform carbon nanotube
film.
[0032] The carbon nanotube segments having a predetermined width
can be selected by using an adhesive tape such as the tool to
contact with the super-aligned array. The pulling direction is
substantially perpendicular to the growing direction of the
super-aligned array of carbon nanotubes.
[0033] More specifically, during the pulling process, as the
initial carbon nanotube segments are drawn out, other carbon
nanotube segments are also drawn out end-to-end due to the van der
Waals attractive force between ends of adjacent segments. This
process of drawing ensures a continuous, uniform carbon nanotube
film having a predetermined width can be formed. The carbon
nanotube film includes a plurality of carbon nanotube segments. The
carbon nanotubes in the carbon nanotube film are all substantially
parallel to the pulling/drawing direction of the carbon nanotube
film, and the carbon nanotube film produced in such manner can be
selectively formed having a predetermined width. The carbon
nanotube film formed by the pulling/drawing method has superior
uniformity of thickness and conductivity over a disordered carbon
nanotube film. Furthermore, the pulling/drawing method is simple,
fast, and suitable for industrial applications.
[0034] Step (c2) can be executed by several methods. A first method
is executed by placing a carbon nanotube film on a surface of the
first electrode and the second electrode along a direction
extending from the first electrode to the second electrode. A
second method is executed by covering at least two carbon nanotube
films stacked with each other and situated such that a preferred
orientation of the carbon nanotubes is set at an angle with respect
to each other. The angle approximately ranges from 0.degree. to
90.degree.. A third method is executed by the following steps:
(c21) supplying a supporting element; (c22) covering at least two
carbon nanotube films stacked on each other and situated such that
a preferred orientation of the carbon nanotubes being set at an
angle with respect to each other to form a carbon nanotube film
structure, the angle approximately ranging from 0.degree. to
90.degree.; (c23) cutting away excess portions of the carbon
nanotube film structure; (c24) treating the carbon nanotube film
structure via an organic solvent; (c25) removing the carbon
nanotube film structure from the supporting element to form a
free-standing carbon nanotube film structure; and (c26) placing the
free-standing carbon nanotube film structure on the surface of the
first electrode and the second electrode. Since the carbon nanotube
film has a high surface-area-to-volume ratio, the carbon nanotube
structure formed by at least one carbon nanotube film may easily
adhere to other objects. Thus, the carbon nanotube film can
directly be fixed on the first electrode, the second electrode, or
the substrate because of the adhesion properties of the nanotubes.
It can be understood that the carbon nanotube structure can also be
secured on the first electrode and the second electrode via
adhesive or conductive glue.
[0035] The carbon nanotube film structure secured on the first
electrode and the second electrode can be treated with an organic
solvent. The carbon nanotube film structure can be treated by
dropping the organic solvent from a dropper to soak the entire
surface of the carbon nanotube film structure or immerging the
carbon nanotube film structure in a container with organic solvent
filled therein. The organic solvent is volatilizable and can be
selected from the group consisting of ethanol, methanol, acetone,
dichloroethane, chloroform, and combinations thereof. In the
present embodiment, the organic solvent is ethanol. After being
soaked by the organic solvent, the carbon nanotube film structure
can more firmly adhere to the surface of the first electrode or the
second electrode due, in part at least, to the surface tension
created by the organic solvent. The specific surface area of the
film is decreased by the treatment. The high mechanical strength
and toughness thereof is still maintained.
[0036] A low-work-function layer can be further formed on the
surface of the carbon nanotube film structure by a sputtering or
vacuum evaporation method. The low-work-function layer is made of
any material capable of inducing the emissions of electrons from
the thermionic electron source at a low temperature, such as
thorium oxide or barium oxide. Electrons in the low-work-function
layer have a lower work function than that in the thermionic
emitter, and can escape from the low-work-function layer at a lower
temperature. Thus, the low-work-function layer can be used to
induce emissions of electrons from the thermionic electron source
at a lower temperature.
[0037] At least one fixing element can be further formed on the
surface of the first electrode and the second electrode by a screen
printing method, offset printing method, electrostatic spraying
method, electrophoresis method, lithography coating method or other
methods such as UV-curing. Two ends of the carbon nanotube film
structure are fixed between the first electrode, the second
electrode, and the fixing elements.
[0038] Referring to FIG. 2, a thermionic electron source 10
acquired by the present method, in accordance with the present
embodiment, includes a substrate 12, a first electrode 14, a second
electrode 16, and a thermionic emitter 18. The first electrode 14
and second electrode 16 are separately located on a surface of the
substrate 12. The thermionic emitter 18 is located between the
first electrode 14 and second electrode 16 and electrically
connected thereto. The thermionic emitter 18 is suspended above the
substrate 12 by the first electrode 14 and second electrode 16. The
thermionic emitter 18 has a film structure.
[0039] Compared to conventional technologies, the method for making
the thermionic electron source provided by the present embodiments
has the following advantages. Firstly, since the carbon nanotube
film structure is formed by at least one carbon nanotube film
pulled from a carbon nanotube array, the method is simple and
low-cost. Secondly, the thermionic electron source adopting the
carbon nanotube film structure prepared by the present embodiment
can acquire a uniform and stable thermal electron emissions state.
Thirdly, since the carbon nanotube film structure and the substrate
are separately located, the substrate will not transfer the energy
for heating the carbon nanotube film structure to the atmosphere in
the process of heating, and as a result, the thermionic electron
source will have an excellent thermionic emitting property.
Finally, since the carbon nanotube film structure has a small width
and a low resistance, the thermionic electron source adopting the
carbon nanotube carbon nanotube film structure can emit electrons
at a low thermal power, thus the thermionic electron source can be
used for high current density and high brightness of the flat panel
display and logic circuits, among other fields.
[0040] Finally, it is to be understood that the above-described
embodiments are intended to illustrate rather than limit the
invention. Variations may be made to the embodiments without
departing from the spirit of the invention as claimed. The
above-described embodiments illustrate the scope of the invention
but do not restrict the scope of the invention.
[0041] It is also to be understood that the above description and
the claims drawn to a method may include some indication in
reference to certain steps. However, the indication used is only to
be viewed for identification purposes and not as a suggestion as to
an order for the steps.
* * * * *